The present invention relates to a wire-dot print head, and more particularly to a wire-dot print head in which a support part of a plate spring is held between a yoke and a spacer.
FIG. 5 is a partially-omitted sectional view of a wire-dot print head 2.
In this wire-dot print head 2, a magnetic flux of a permanent magnet 21 flows through a closed magnetic path consisting of an annular intermediate yoke 22, an annular spacer 23, a plate spring 24, a front yoke 25, an armature 26, a core 27a of an electromagnet 27 and a disk-shaped rear yoke 28, and then returns to the permanent magnet 21. When the electromagnet 27 is not energized the armature 26 mounted to the inner or free end 24a of the plate spring 24 is attracted to the core 27a of the electromagnet 27.
When the electromagnet 27 is energized to generate a magnetic flux in the core 27a opposite in direction to the magnetic flux from the permanent magnet 21, the armature 28 is released from the core 27a, and because of the reaction force of the spring 24, a print wire 18 fixed to the tip of the armature 26 is projected forward (upward as seen in the figure) from a guide frame 30 so as to press an ink ribbon IR and a printing media PM onto a platen PL thereby to transfer the ink from the ink ribbon IR onto the printing media, effecting printing of one dot.
The annular support part 24b of the plate spring 24 is in abutment, at its rear (lower as seen in the figure) surface, with the spacer 23, and, at its front surface, with the yoke 25, and is held between them, and fixed to the intermediate yoke 22 by means of a screw 31.
In another wire-dot print head shown in FIG. 6, the print wire 29 is driven in the same way, but the plate spring 24 is fixed in a different manner. That is, the support part 24 of the plate spring 24 is in abutment, at its rear and front surfaces, with a spacer 23 and a front yoke 25, respectively, and they are bonded by laser welding or the like. This assembly as well as other components such as the permanent magnet are together pressure-held by a clamp spring 32.
In both of the manner of fixing by means of the screw and the manner of bonding by means of laser welding or the like, the support part 24b of the plate spring 24 is held, by abutment between flat spacer 23 and yoke 25, as shown in FIG. 7, so if the flatness of the abutting surfaces of the spacer 23 and the yoke 25 is poor, local stress is created, and the spring force of the plate spring 24 varies. Moreover, the torque in the state in which the screw 31 is tightened can vary and the screw 31 can become loose. Furthermore, variation in the position of bonding can vary the spring constant of the plate spring, and the spring force varies.
In addition, as shown in the sectional schematic view of FIG. 8, because the screw 31 tightens the spacer 23 at about the mid-point, as shown in FIG. 9 showing how the forces act, reaction forces R.sub.2 and R.sub.1 are created at the central position of the screw 31, and at the inner periphery of the spacer 23, respectively. If the load on the plate spring is F, the distance between the positions of the load F and the reaction force R.sub.1 is L, and the distance between the positions of the reaction forces R.sub.1 and R.sub.2 is 1.sub.1, then: EQU R.sub.1 l.sub.1 =F.multidot.(l.sub.1 +L) EQU R.sub.2 l.sub.1 =F.multidot.L
So, EQU R.sub.1 =F.multidot.(l.sub.1 +L)/l.sub.1 EQU R.sub.2 =F.multidot.L/l.sub.1
In the case of welding by laser or the like, the reaction forces R.sub.1 and R.sub.2 are given by similar equations, as will be understood from FIG. 11. That is, the distance 1.sub.1 between the positions of the reaction forces, i.e., the center of bonding or fixing, and the inner periphery 23a of the spacer cannot be sufficiently long. In order to provide a constant spring force F at the time of attraction, the reaction forces R.sub.1 and R.sub.2 must be large. This is not satisfactory from the viewpoint of spring operation.
A proposal has been made to provide free support for the support part of the plate spring at two support points so that the entire length of the spring is the effective length of the plate spring (Japanese Patent Application Kokai Publication No. 126172/1983). In the configuration of this proposal, the relative position between the two support points and the tip of the print wire and the part of the platen on which the print wire impacts is so determined that in the state in which the tip of the print wire is in contact with the surface of the platen, the plate spring is supported and bent. If however there is an error in the relative position between the print wire and the platen, the spring force varies, and the plate spring can get out of the two support points. The above structure is therefore not fully satisfactory.